My invention pertains to a fail-safe resistor. More particularly, the invention relates to a four terminal resistor for use in vital circuitry to assure a safe failure if the resistor opens.
In general, resistors are considered safe circuit elements since the usual failure is an open circuit fault which interrupts operation of the apparatus. However, in some electrical circuits using resistors, e.g., voltage divider networks, the breaking of a resistor, due to mechanical forces or chemical corrosion, may cause the output signal supplied to the next circuit element or a feedback signal to increase in value. This at least causes an incorrect operation of the circuit arrangement. More seriously, the fault may create an unsafe condition, particularly in railroad signaling or control apparatus. Thus a requirement exists that some arrangement be provided to cancel or inhibit an improper output. Special circuit connections are possible and have been used but only with extra circuit complications and additional cost.
Accordingly, an object of my invention is an improved, vital resistor which interrupts the circuit connections and assuredly cancels the output signal if the resistor breaks.
A further object of the invention is a four terminal resistor so constructed that any structural break will interrupt the continuity of the circuit arrangement in which used.
Another object of the invention is a fail-safe resistor formed by resistance material deposited between two terminal connector strips deposited on a fragile support block with terminals at each end of each strip, structured such that any breakage of the resistance material or the support block opens one or both connector strips to interrupt the circuit between input and output terminals.
Other objects, features, and advantages of the invention will become apparent from the following specification and claims when taken with the accompanying drawings.